There are several known electrophotographic methods for the reproduction of images. Typically in such processes, a latent image is formed in a photosensitive material, then developed and transferred to a final receptor surface, e.g., a paper sheet.
In one technique, known as persistent conductivity, the latent image comprises an imagewise pattern of non-conductive and conductive regions in the imaging layer of a persistent conductivity-type recording member. Such recording members typically have a multi-layered construction. The first layer, sometimes called a base layer, is non-conductive, and typically comprises, for example, paper, polypropylene, polyester terephthalate, etc. On a side of the first layer, is the imaging or photoconductive layer, typically comprising a photosensitive material dispersed in a resinous binder material. Examples of commonly used photosensitive materials include zinc oxide, lead sulfide, cadmium sulfide, selenium or combinations thereof. An example of a resinous binder material commonly used in persistent conductivity-type recording members is styrene/butadiene copolymer. In some instances, the persistent conductivity-type recording member may further comprise a conductive layer or coating disposed on the underside of the nonconductive layer, i.e., the side opposite that which the photoconductive layer is on. Such conductive layers are intended to provide a more effective ground potential to the back side of the recording member, thereby improving the resolution of images formed therewith via a persistent conductivity process.
In a persistent conductivity process, image reproduction is achieved by exposing the recording member, which is in an initially substantially uncharged and dark condition state, to light in imagewise fashion, thereby increasing the conductivity of the light-struck portions of the recording member. Development of the latent image is then achieved by application of toner to the surface of the recording member under the influence of an electric field tending to attract the toner to the surface of the recording member and a magnetic field tending to attract the toner back to the toner station. The toner deposited on the light-struck portions of the recording member loses at least some of its induced charge due to the conductivity of those portions of the sheet and is removed by the magnetic field. The toner on the non-light-struck portions of the recording member substantially retains its charge because of the relatively non-conductive nature of the non-light-struck portions of the sheet, such that the effect of the electric field upon the induced charge exceeds the strength of the magnetic field thereby causing the toner to remain on the non-light-struck portions of the recording member. The imagewise deposit of toner on the surface of the recording member is typically then transferred to a final receptor such as a paper sheet or an intermediate receptor such as a web for subsequent transfer to the final receptor. Such a process is taught by Shely in U.S. Pat. Nos. 3,563,734 and 3,764,313. Thus it is seen that the latent image, i.e., region of increased conductivity, of a recording member used in such a recording process must remain, i.e., "persist", at least long enough to be developed. The latent image should resist reverting to its dark, i.e., nonconductive state, when subjected to the electric field of the toner station, and should quickly dissipate the induced charge from the toner so as to reduce the tendency for toner to be deposited in the background. Also, the recording member should be such that an effective differential in conductivity is achieved between the light-struck and non-light-struck portions thereof such that good toner deposition is achieved in the desired regions while the background remains substantially clear.
The construction and properties of persistent conductivity-type recording members typically differ from those of recording members used in other electrophotographic reproduction processes. For instance, in xerography, the photoconductive layer is on a conductive layer, e.g., an aluminum film or a paper sheet having a conductive coating thereon. Thus, the photoconductive layer is disposed directly on a conductive layer, contrary to the construction of a recording member for use in a persistent conductivity process. The recording member may further comprise a base layer on the opposing side of the conductive layer to strengthen the mechanical properties of the recording member. In the xerographic process, the latent image is formed by charging the xerographic recording member with a substantially uniform static charge, e.g., by exposure to a corona discharge, and then exposing the member to light, thereby increasing the conductivity of the recording member in the light-struck areas and causing the charge in those areas to be reduced whereas the non-light-struck areas substantially retain their initial charge. Such latent images are then developed by depositing toner on the surface of the recording member. Toner is attracted to the non-light-struck areas of the recording member which substantially retain their initial charge, whereas the light-struck areas from which the initial charge has been bled off, substantially do not attract toner. The imagewise deposited toner can then be transferred to a receptor surface.
It has been taught that binders suitable for use in recording members used in xerography processes are not necessarily useful for use in persistent conductivity processes. For instance, U.S. Pat. No. 3,010,884 (Johnson et al.) discloses that a certain binder used in xerographic recording processes is not useful in a persistent conductivity process (column 2, lines 38-58).
In order to be suitable for use in the photoconductive layer of a persistent conductivity-type electrophotographic recording member, a binder must possess several physical and electrical characteristics. It must have good insulative properties so that the resultant photosensitive material/binder dispersion will have suitable electrical resistivity, dielectric constant, and dielectric strength. It should not provide a short circuit current path between the particles of photosensitive material, nor should it impede the current flow between such particles. The binder should eliminate parallel leakage paths around the particles caused by absorbed moisture. It should be solvent soluble and exhibit the ability to be wet and disperse, but not react with, particles of photosensitive materials to form a tough, flexible, non-tacky film that preferably has a smooth finished surface and is moisture resistant.
Many resins require special processing in order to be used as a binder for a photoconductive film. A polymer may contain contaminants or deleterious by-products from its manufacture, such as traces of salts or metals, which must be treated or removed before the resin will function satisfactorily. These contaminants, which may reduce the humidity resistance and alter the dielectric properties of the film, are typically removed by rinsing the resin flocculent with water. The remaining water must in turn be removed because it may interfere with proper formation of the photoconductive film. For instance, water typically causes zinc oxide particles to become tacky, thereby making it more difficult to uniformly disperse the photosensitive material throughout the binder, and tends to render the resultant film conductive, thereby forming an undesired image in those areas of the film which contain water.
One known binder, a copolymer of styrene and butadiene, is commonly used in conjunction with zinc oxide photoconductor systems. Forming a photoconductive film with that binder, which is typically blended with the photosensitive material in a ball mill mixer, is often difficult because it frequently exhibits wide variations in viscosity, dielectric characteristics, molecular weight distribution, and purity, as well as other critical functional properties. In order to achieve convenient replicatable performance with a ball mill mixer, the viscosity of the binder/photosensitive material mixture must be substantially consistent from batch to batch. If the viscosity is too high, the ball milling must be continued for a longer period, which tends to disturb the particle size distribution of the photosensitive material. As the particles of photosensitive material are ground into smaller size, the photospeed of the recording member may be enhanced, but at the expense of lower contrast. If the viscosity is too low, the photosensitive material may form agglomerates, rather than evenly disperse, thus rendering the resultant film difficult to photosensitize.
A coating composition containing the styrene/butadiene copolymer may also be prepared with a homogenizer, but the binder must first be homogenized alone to lower its viscosity which is typically too high and to break up any gel particles before the photosensitive material is added and the mixture again passed through the homogenizer. The binder must be homogenized before the photosensitive material is added because the high homogenization pressures necessary to lower its viscosity and break up any gel particles will typically break down the particles of the photosensitive material thereby interfering with the photosensitive properties of the resultant film.